1. Field of the Invention
This invention relates to a method for the production of high-purity monoethylene glycol. More particularly, it relates to a method for producing a high-purity monoethylene glycol possessing a fiber-grade quality and excelling in thermal stability by using as the raw material for a hydration reaction an unpurified crude ethylene oxide obtained during the course of producing and purifying ethylene oxide by catalytic vapor phase oxidation of ethylene with a molecular oxygen-containing gas in the presence of a silver catalyst.
2. Description of the Prior Art:
Among the conventional methods for producing ethylene glycol by using as the raw material for a hydration reaction an unpurified crude ethylene oxide obtained during the course of manufacture of ethylene oxide by catalytic vapor phase oxidation of ethylene with a molecular oxygen-containing gas in the presence of a silver catalyst, there have been counted (1) a method which as described in JP-B-61-3,772 comprises condensing the vapor emanating from the top of an ethylene oxide stripper in two condensers, refluxing the condensate to the stripper, forwarding the uncondensed gas to a third condenser and condensing the gas therein, forwarding the resultant condensate to a hydration reaction vessel, and carrying out a reaction of conversion to glycol, (2) a method which as described in U.S. Pat. No. 3,904,656 comprises guiding a gas emanating from the top of a stripper to a reabsorption column, performing therein absorption of the gas with an absorbing liquid having water as a main component thereof, forwarding the absorbing liquid having absorbed ethylene oxide to a hydration reaction vessel, and carrying out a reaction of the absorbing liquid, and (3) a method which as described in U.S. Pat. No. 3,964,980 comprises condensing the gas arising from the top of a stripper by a condenser, returning the condensate as reflux to the stripper, guiding the uncondensed gas to a reabsorption column to be absorbed therein, guiding the absorption liquid having absorbed the produced ethylene oxide to a hydration reaction vessel to be subjected to reaction therein.
U.S. Pat. No. 4,349,417 discloses a method for the production of high-purity monoethylene glycol, characterized by adding an aqueous alkalimetal compound solution to the reaction product between a hydration reaction vessel and a monoethylene glycol rectification column adapted to distil monoethylene glycol through the top thereof and effecting this addition of the reaction product in an amount such that the reaction product, while entering the monoethylene glycol rectification column, assumes a pH value in the range of 7-10. In U.S. Pat. No. 4,349,417 mentioned above, in disclosing the invention, makes no mention of the fact that the ethylene oxide (crude ethylene oxide) as the raw material for the hydration reaction incorporates such organic chlorine compounds as ethylene dichloride and ethylene chlorohydrin as extraneous matter. It discloses absolutely no knowledge concerning the technical problem which arises from the use of the raw material for the hydration reaction containing such organic chlorine compounds and the measure for the solution thereof.
We have proposed as in JP-B-61-3,772 a method for producing high-purity monoethylene glycol by using as the raw material for a hydration reaction the unpurified crude ethylene oxide obtained during the course of manufacture of ethylene oxide by catalytic vapor phase oxidation of ethyene with a molecular oxygen-containing gas. This method implements the manufacture of monoethylene glycol without requiring addition of an alkali because the process involved therein has no need for consideration about a measure to cope with the organic chloride compounds arising from the chlorine compound in the crude ethylene oxide.
No method has been heretofore proposed to date which produces high-purity monoethylene glycol by using as the raw material for a hydration reaction an unpurified crude ethylene oxide containing organic chloride compounds. Such is the true state of prior art.
When the hydration reaction is carried out with the liquid which is obtained by condensing the gas arising from the top of an ethylene oxide stripper, refluxing the condensate, and cooling or absorbing the uncondensed gas as contemplated by the conventional method, the ethylene oxide obtained from the uncondensed gas at the first stage does not include most of chloride compounds because the chlorine compounds migrate into the condensate at the first stage or to the bottoms of the column. It may well be concluded, therefore, that the ethylene glycol to be obtained by hydrating this ethylene oxide contains such chlorine compounds only in an amount deserving to be called xe2x80x9ctrace.xe2x80x9d
We, not satisfied with the prior art and with a view to further simplifying the process for the production of monoethylene glycol and saving the energy consumed therein, have studied the feasibility of using as the raw material for the hydration reaction the unpurified crude ethylene oxide obtained without refluxing the top of an ethylene oxide stripper during the manufacture of ethylene oxide by catalytic vapor phase oxidation of ethylene with a molecular oxygen-containing gas in the presence of a silver catalyst. We have consequently found that when the unpurified crude ethylene oxide is used as the raw material for the hydration reaction, the monoethylene glycol product consequently obtained offers no sufficient stability to withstand the heating and could not afford a fiber grade product.
Specifically, during the manufacture of ethylene oxide by catalytic vapor phase oxidation of ethylene with a molecular oxygen-containing gas in the presence of a silver catalyst, such organic chloride compounds as ethylene dichloride and ethylene chlorohydrin emanate from the top of an ethylene oxide stripper in conjunction with ethylene oxide and the like. It has been found that notwithstanding such organic chlorine compounds are removed by a subsequent operation of separation by distillation, the monoethylene glycol obtained by using the condensate obtained from the gas emanating from the top of the ethylene oxide stripper in its unmodified form as the raw material for the hydration reaction is destined to entrain as a foreign matter the chlorine compounds in an amount corresponding to 10-20% of the organic chlorine compounds which are present in the raw material for the hydration reaction.
We, therefore, have made a diligent study with a view to elucidating the cause for this persistent adulteration of the monoethylene glycol and have been consequently ascertained that the monoethylene glycol as a finished product has such qualities as thermal stability degraded because the inorganic chlorine originating in the organic chlorine compounds such as ethylene dichloride which are contained in the unpurified crude ethylene oxide (which are added during the process for the production of ethylene oxide by catalytic vapor phase oxidation of ethylene with a molecular oxygen-containing gas in the presence of a silver catalyst) emerges in the hydration reaction solution and, consequently, such impurities as ethylene chlorohydrin are formed during the process of dehydration of ethylene glycol and are suffered to mingle as foreign matter into the rectified monoethylene glycol.
We have studied from various angles the process for the production of monoethylene glycol with a view to simplifying the process of production, saving the energy spent therein, and reducing the cost of production and have been consequently ascertained that in the conventional technique for producing monoethylene glycol by using as the raw material for the hydration reaction of purified ethylene oxide, the objects mentioned above are accomplished by changing the raw material for the hydration reaction from purified ethylene oxide to unpurified crude ethylene oxide but that this measure not merely fails to bring a sufficient reduction in the concentration of heavy aldehyde, particularly glycol aldehyde, which is entrained by the crude ethylene oxide, formed by the reaction of hydration, and brought eventually into the monoethylene glycol product to stay therein but also degrades the thermal stability of the monoethylene glycol as a finished product. This degradation of the thermal stability is considered to be caused not only by the glycol aldehyde but also by the impurities (such as, for example, ethylene chlorohydrin and diethylene glycol monochlorohydrin) which occur when organic chlorine compounds, aldehydes, and organic acids entering the site of process owing to the use of the crude ethylene oxide are varied during the hydration reaction and completed through the reaction of monoethylene glycol with a substance having a boiling point approximating closely thereto or during the course of the rectification of monoethylene glycol by distillation.
An object of this invention, therefore, is to provide a method for the production of high-purity monoethylene glycol which succeeds in simplifying the process and operation of the production, saving the energy to be spent in the production, and reducing the cost of the production, decreasing the glycol aldehyde content in the monoethylene glycol to be produced and, at the same time, decreasing the substances causing degradation of the thermal stability mentioned above, and satisfying the qualities required of the fiber grade high-purity monoethylene glycol.
Another object of this invention is to provide a method for the production of a high-purity monoethylene glycol which succeeds in simplifying the process and operation of the production, saving the energy to be spent in the production, repressing the inclusion of such impurities as ethylene chlorohydrin as foreign matter into the monoethylene glycol to be obtained as a product, and satisfying the product specifications required of a fiber grade high-purity monoethylene glycol.
Still another object of this invention is to provide a method for the production of a fiber grade high-purity monoethylene glycol product which, in spite of the use of an unpurified crude ethylene oxide containing chlorine compounds as the raw material for a hydration reaction, represses the inclusion of impurities as a foreign matter similarly to the monoethylene glycol product obtained by using purified ethylene oxide as the raw material for the hydrolygic reaction and manifests high thermal stability.
We have pursued a diligent study concerning methods for the production of high-purity monoethylene glycol with a view to accomplishing the objects mentioned above and have consequently discovered that by using unpurified ethylene oxide as the raw material for a hydration reaction thereby simplifying the process and operation of the production, saving the energy to be consumed in the production, and lowering the cost of production and, at the same time, adopting a very simple means of introducing the technique of side cut to a monoethylene glycol rectification column without additionally installing devices for the removal of impurities, improving such devices, and adding new steps, it is made possible unexpectedly to acquire from the side cut part a high-purity monoethylene glycol product containing glycol aldehyde at an extremely low concentration and manifesting high thermal stability and accomplish the simplification of the process and operation of the production, saving the economy to be spent therein, and lowering the cost of production fully satisfactorily as a whole. This invention has been perfected as a result.
We have continued a diligent study concerning methods for the production of high-purity monoethylene glycol with a view to accomplishing the objects mentioned above. We have consequently discovered that by using the chlorine compound-containing unpurified crude ethylene oxide obtained without refluxing the top of an ethylene oxide stripper as the raw material for a hydration reaction thereby promoting the saving of the energy to be spent in the reaction and, at the same time, fixing the inorganic chlorine arising from decomposition of part of an organic chlorine compound from an external source with an alkaline substance by extremely simple means without requiring installation of a complicate treatment or apparatus or increasing the number of steps greatly with respect to a new addition of impurities originating in the unpurified crude ethylene oxide and thereafter purifying the monoethylene glycol in a monoethylene glycol rectifying column, it is made possible to repress the formation of such an organic chlorine compound as ethylene chlorohydrin as an impure substance and prevent this defiling compound from mingling into the product. This invention has been perfected as a result.
The objects mentioned above are accomplished by a method for the production of monoethylene glycol of high purity, which comprises subjecting ethylene to catalytic vapor phase oxidation with a molecular oxygen-containing gas thereby obtaining an ethylene-containing gas, exposing the ethylene oxide-containing gas to an absorbing solution thereby obtaining an ethylene oxide-containing solution, stripping the ethylene oxide-containing solution in a stripper, condensing the vapor emanating from the top of the stripper thereby obtaining crude ethylene oxide, subjecting at least part of the crude ethylene oxide to a hydration reaction, adding an alkaline substance in an amount of not less than 0.5 atomic equivalent weight to the chlorine atom contained in the hydration reaction solution, and subjecting the resultant mixture to rectification.
The objects mentioned above are also accomplished by a method for the production of monoethylene glycol of high purity, which comprises subjecting ethylene to catalytic vapor phase oxidation with a molecular oxygen-containing gas thereby obtaining an ethylene-containing gas, exposing the ethylene oxide-containing gas to an absorbing solution thereby obtaining an ethylene oxide-containing solution, stripping the ethylene oxide-containing solution in a stripper, condensing the vapor emanating from the top of the stripper thereby obtaining crude ethylene oxide, subjecting at least part of the crude ethylene oxide to a hydration reaction, dehydrating the product of the hydration reaction, introducing the side cut technique to a monoethylene glycol rectification column, and acquiring monoethylene glycol from the side cut part.
The objects mentioned above are accomplished by a method for the production of monoethylene glycol of high purity, which comprises subjecting ethylene to catalytic vapor phase oxidation with a molecular oxygen-containing gas thereby obtaining an ethylene-containing gas, exposing the ethylene oxide-containing gas to an absorbing solution thereby obtaining an ethylene oxide-containing solution, stripping the ethylene oxide-containing solution in a stripper, condensing the vapor emanating from the top of the stripper thereby obtaining crude ethylene oxide, subjecting at least part of the crude ethylene oxide to a hydration reaction, adding an alkaline substance in an amount of not less than 0.5 atomic equivalent weight to the chlorine atom contained in the hydration reaction solution, dehydrating the product of the hydration reaction, introducing the side cut technique to a monoethylene glycol rectification column, and acquiring monoethylene glycol from the side cut part.
The method of this invention for the production of monoethylene glycol of high purity succeeds in simplifying a process and saving energy by using the unpurified crude ethylene oxide containing a chlorine compound as the raw material for a hydration reaction, preventing inorganic chlorine from reacting with ethylene glycol by adding an alkaline substance in a prescribed amount to the hydration reaction solution and consequently fixing the inorganic chlorine, exalting greatly the transmittance to the ultraviolet ray of the rectified monoethylene glycol after application of heat and acquiring a monoethylene glycol product of high purity of fiber grade enjoying high thermal stability by repressing the formation of an organic chlorine compound during the purification of monoethylene glycol.
Further, the method of production of this invention is economically at an advantage in only requiring the existing production facility to incorporate therein a line for the addition of the alkaline substance and making effective use of the production facility in its existing state.
This invention has another advantage of successfully economizing the process for the production of monoethylene glycol in terms of the consumption of energy (allowing more saving of the energy than when the top of the ethylene oxide stripper is refluxed) by using as the unpurified crude ethylene oxide at least one of the oxides, i.e. (1) the ethylene oxide in a condensate obtained by cooling and/or absorbing the ethylene oxide-containing vapor emanating from the top part of the ethylene oxide stripper thereby condensing the vapor partly or wholly, (2) extracting the condensate obtained by partly or wholly condensing the ethylene oxide-containing vapor emanating from the top part of the ethylene oxide stripper without refluxing the condensate to the stripper, and (3) the product obtained by partly condensing the ethylene oxide-containing vapor emanating from the top part of the ethylene oxide stripper thereby obtaining a first condensate, guiding the remaining uncondensed gas, either after being further condensed or in its unmodified state, to the ethylene oxide dehydration column, and mixing the resultant dehydrated ethylene oxide with the first-condensate mentioned above.
This invention accomplishes simplification of the process and operation of the production, reduction in the consumption of energy, and decrease of the cost of production by introducing the side cut technique to the monoethylene glycol rectification column and acquiring monoethylene glycol from the side cut part in producing monoethylene glycol by using the unpurified crude ethylene oxide as the raw material for the hydration reaction, permits such contaminants as glycol aldehyde and substances detrimental to thermal stability which have been separated with great difficulty from monoethylene glycol by the conventional treatment of distillation to be condensed from the top part of the monoethylene glycol rectification column and discharged from the system, represses to an exceptionally great extent the amount of such contaminants otherwise possibly suffered to mingle into ethylene glycol extracted from the side cut part. Thus, the produced monoethylene glycol enjoys extremely high purity and high yield and realizes a generous addition to value as a fiber grade product of high thermal stability.